The present invention relates to a color image sensor in which each of a multiplicity of picture elements comprises a plurality of photoelectric conversion elements one for each of the three primary colors, and in which color signals from the photoelectric conversion elements of the same picture element are simultaneously extracted to be added by color.
In the recent color printing art, color compensating filters are used to control the proportion of the primary color components of printing light for color correction in order to obtain well-balanced color prints. This control is carried out according to several groups of scenes into which color originals (positives or negatives) are classified based on their color characteristics. This classification of the color originals is effected in accordance both with the large area transmittance density (LATD) for each primary color (blue, green and red) component of light and with the densities of points on the color original for each primary color component of light. The LATD of the color original is obtained by measuring the light transmitted through the color original by an averaged light measuring method. The color originals whose LATDs for the three primary colors are substantially constant are used as standards. When printing from such standard color originals, the color compensating filters are adjusted to control the color components of light so that the light transmitted through the color original becomes gray as a result of integration, and this is known in the art as a gray integration printing method (LATD method).
On the other hand, the color originals of which any one of the LATDs for the three primary colors is abnormally different from the other are deemed color failure originals. Such color failure originals are classified based on their characteristics such as tints of their picture elements, the relationships between the locations thereof and the tints (the balance between the densities for the three primary colors) and the like into small groups, by using a fluorescent lamp, a tungsten lamp, having low color temperatures, having high color temperatures, having a change on standing, etc. For such color failure originals, the color compensating filters are adjusted according to the kinds of small groups to assure proper color correction.
For the classification of color originals, there is used in color printers a photographic density information gathering device which is adapted to measure the tints of 100 to 200 picture elements into which the scene of each color original is divided. Accordingly, it is important in this measurement to avoid the occurrence of a color registration error.
As image sensors of the type which stores charges produced by photoelectric conversion elements so that a large output can be extracted, there are known CCD-type, MOS-type and CPD-type, solid state imaging devices. In addition, there are known storage type color image sensors which comprise photoelectric conversion elements integrated with color filters of blue, green and red arranged on a single plate. When using this single-plate image sensor for measuring the primary color components of light, because all of the photoelectric conversion elements measure the transmitted light through different points of the color original, an unavoidable color registration error occurs. A way of solving the color registration error problem is to provide that each picture element comprises a plurality of photoelectric conversion elements one for each primary color (blue, green and red) and then to extract the color signals from the photoelectric conversion elements in sequence by color, and finally to add up the color signals of the same picture element by color.
There is, however, a problem in such color image sensors that, since signals from the color image sensor are extracted in a time series for every horizontal row of photoelectric conversion elements, there is the necessity to provide an analog data memory which can simultaneously store color signals extracted from a plurality of horizontal rows of photoelectric conversion elements and an adder for adding up the color signals from different rows and columns of photoelectric conversion elements. This provision of the analog data memory and the adder makes the color image sensor complicated.
As is described in, for example, Japanese Kokai No. 59-54384, it is known that the dynamic range of an image sensor can be expanded by changing its storing period (light converions period). Therefore, in color image sensors, low-noise color signals can be obtained by measuring the primry color components of light for different storing periods by color. In the above-described conventional color image sensors, however, it is impossible to change the storing period by color. The reason is that signals of the three primary colors from the image sensor are extracted mixed, from each horizontal row of photoelectric conversion elements. Furthermore, when considering the logarithmic transformation of color signals, and the stable extraction of color signals which is achieved by operating the system using the color image sensor with some sufficient allowance for operational timing, and the like, it is desirable to perform the extraction of color signals as slowly as possible. Such slow signal extraction makes it possible to utilize inexpensive signal processing circuits with low operation speeds, thereby decreasing the manufacturing cost of the system. For these reasons, it is preferable that the storing period for signal charges and the initiation of signal extraction can be determined independently for each color.